Atomic force microscopy (AFM) is a scanning microscopy technique developed in the 1980s which can be used to obtain atomic scale resolution. In contrast to scanning tunneling microscopy, atomic force microscopy is not limited to the formation of images of conductive surfaces, which renders it particularly suitable for insulating materials, semiconductors, as well as samples of a biological nature. This technique has applications in many fields, such as the microelectronics industry or in biology. The essential component of a conventional atomic force microscope is a probe constituted by a lever connected to a support end, the opposite end being provided with a tip orientated towards the surface or the object to be observed. The lever is generally of the order of a few tens or hundreds of micrometers in length, and the tip has a radius of curvature of a few tens of nanometers. Such a probe, generally constituted by silicon, may be manufactured using conventional photolithographic techniques. When the tip of the probe is brought close to a surface, it is influenced by attractive or repulsive forces of a chemical, van der Waals, electrostatic and/or magnetic nature. By measuring those forces while the tip scans the surface of the object to be observed, it is possible to reconstitute an image of the surface. The forces being exerted between the tip and the object can be measured in different manners. The simplest and oldest technique (static AFM) is limited to observing, in particular by optical means, the deflection of the mounted lever supporting the tip. Such optical means typically comprise a laser diode which illuminates a reflective surface of the lever at an oblique incidence, a detector which is sensitive to the position of the reflected beam which it receives and thus is capable of detecting modifications in the orientation of the beam due to deflections of the lever. The sensitivity may be further improved by vibrating that lever in one of its fundamental modes, and by observing the variations in resonance frequency induced by the force gradients (dynamic AFM). That vibration is obtained by means of a piezoelectric tube connected to a support.
Standard tips are conventionally conical in shape, but it should be understood that this type of tip can only be used to explore reliefs without overhangs, such as hill and valley shapes.
A known solution to that problem consists of using tungsten tips with complex shapes known as CD (critical dimension) tips that can be used to measure the dimensions of complex reliefs, in particular reliefs with overhangs. FIG. 1a shows, by way of example, the principle of examining a relief 1 without an overhang using a simple conical tip 2. FIG. 1b illustrates the difficulty arising when examining a shape 3 with cavities or overhangs using that tip 2, which cannot touch the zones 4 below the overhangs. FIG. 1c represents the principle of examining a relief having overhangs with the aid of a flared CD tip 5 with a complex shape (elephant's foot shape with a flared end so as to be able to touch the relief below the overhang). The structure of the flared tip 5 means that complex reliefs with overhangs can be explored.
For simple tips and for complex tips, a problem arises regarding integral characterization (shape and dimensions) of the tip used. That characterization step is fundamental to the accuracy and reproducibility of the measurements.
In known manner, the characterization of a CD tip with a complex shape enabling the characterization of objects in three dimensions is carried out using two distinct characterization structures formed from silicon, one enabling the overall diameter of the tip to be determined, and the other enabling its shape to be determined.
FIGS. 2a to 2d illustrate the manner by which the overall diameter of a flared tip 10 with a complex shape can be determined. This tip 10 with a complex tip comprises two projecting lateral tips 11 and 12 and has a generally circular or elliptical section. The overall diameter of the tip 10 corresponds to the width L2 separating the two lateral tips 11 and 12 projecting on either side; in other words, the overall diameter of the tip corresponds to the largest diameter of the set of horizontal sections of the tip 10. The first structure 13 enabling the diameter of the tip to be determined is constituted by a line 14 of silicon having relatively smooth vertical flanks rising above a silicon surface 15. The characterization structure 13 can also be denoted by the acronym VPS (vertical parallel structure). The width L1 of the line 14 of this VPS structure has been pre-calibrated, and so it can be used to determine the overall diameter of the tip. Knowing the dimension L1 of the line 14, if the structure 13 is scanned with the tip 10 with a complex geometry, then after the measurement, a line 17 is obtained with a virtual size L (see FIG. 2b) which is the sum of the width L1 of the line 14 and the actual width of the tip L2. This is known as “convolution” of the tip 10 with the calibration structure 13 (i.e. as soon as a probe is used to measure the dimension of any object, the dimension of the outer envelope of the probe is integrated into the measurement). The displacement contour followed by the tip 10 is thus a rectangle with a width L that is not L1 but is L1+L2. Thus, the absolute size of the tip can be deduced: L2=L−L1. If the point of contact P1 between the tip 10 and the structure 13 at a height Z1 (see FIG. 2c) is considered, the x coordinate recorded is the coordinate x1 corresponding to a shift of half a diameter, L2/2, with respect to the edge of the structure (the origin of the reference, or reference point, of the AFM measurement is in fact the centre C of a circular or elliptical section passing through the two projecting tips 11 and 12). For reasons of symmetry, the same applies on the other side of the structure at the same height Z1. Thus, by scanning the whole calibration structure 13 along the x and z axes, after the measurement is complete (see FIG. 2d), the rectangular shape of the virtual line 17 shown in FIG. 2b is obtained, corresponding to convolution of the tip 10 with the calibration structure 13 for which the final width obtained is L=L1+L2. Thus, the diameter of the tip can be obtained: L2=L−L1.
FIGS. 3a to 3c illustrate the manner of determining, imaging and characterizing the left and right hand sides of a flared tip 10 with a complex geometry and thus of gaining access to the shape of this tip in a quantitative manner using characteristic variables and employing a second characterization structure 18 as shown in FIG. 3a. In order to characterize a tip, it is very important for the various regions of the tip 10 to be in contact with the characterization structure 18. In the ideal case, this contact point will be unique for each of the sides of the tip 10. The key step thus lies in the level at which the second contact points are located, to enable integral characterization of the geometry of the tip to be carried out. In order to obtain two quasi-point contact points, the edges 19 and 20 of the characterization structure 18 are slightly thinned in order to obtain radii of curvature of less than 10 nm (see FIG. 3a).
FIG. 3b represents the respective right and left hand portions of two characterization structures 18 disposed one beside the other and forming a cavity 21. The structure 18, termed an IFSR (isolated flared silicon ridge) structure, has a vertical re-entrant profile. As indicated above, to characterize a tip, it is very important that various regions of the tip 10 are in contact with the characterization structure 18. Ideally, this contact point will be unique for each of the sides of the tip. The key step thus lies in where the two contact points between the structure and the tip occur which will allow integral characterization of the geometry of the tip. In order to obtain two quasi-point contact points, the edges 19 and 20 of the structures 18 are slightly raised and thinned in order to acquire radii of curvature of less than 10 nm (see FIG. 3b). The contour followed by the tip 10 when it is displaced means that the shape of the tip can be determined (by deconvolution with the shape of the cavity and its overhangs). The reconstitution of the shape of the tip is illustrated in FIG. 3c. A specific contact point pi (i varies from 1 to 3 in the example of FIG. 3c) between the tip and the end 19 of the characterization structure 18 will correspond to a pair of coordinates (xi, zi). By passing the tip 10 over the entire characterization structure 18, it is then possible, by representing all of these coordinate pairs, to determine the shape of the left hand side of the tip 10 (see curve 22). The same operation is carried out symmetrically on the right hand side of the tip 10. Thus, the shape is reconstituted by determining a succession of coordinates (xi, zi) for the contact points as the tip is displaced in the cavity 21, and it is the curve formed by this succession of coordinates that constitutes the data for deconvolution. The end 19 or 20 of the part ascending of the characterization structure 18 allows contact between it and the tip, and so the end 19 or 20 must as a consequence by extremely thin so that contact is as point-like as possible. Without this, good quality accuracy and reproducibility of the measurement cannot be achieved.
However, it will be noted that the characterization structures mentioned above suffer from a certain number of disadvantages when characterizing tips for atomic force microscopy, AFM. In fact, a characterization structure as illustrated in FIGS. 3a to 3c comprises projecting tips (formed by the ends 19 and 20), these tips being thinned at their ends in order to acquire radii of curvature of less than 10 nm, i.e. less than the radii of curvature of the tips to be characterized. In passing the tip to be characterized over the thinned precursors 19 and 20 in order to determine the shape of the tip to be characterized, these tips are eroded. This erosion phenomenon means that the tips 19 and 20 have to be thickened such that the radii of curvature of the thinned tips get to be more than 10 nm, i.e. larger than the radius of curvature of the tip to be characterized, and it is no longer possible to acquire sufficient measurement points that are characteristic of the diameter of the tip to be characterized. For this reason, such structures comprising eroded tips are no longer capable of properly characterizing tips for atomic force microscopy, AFM.